(1) Field of the Invention
The present invention relates to a process for producing alkali metal polymaleimide salts by alkaline hydrolysis of maleimide polymers prepared by polymerization of maleimide monomers in the presence of a metal oxide and alcohol initiator or a base. In particular, the process produces alkali metal polymaleimide salts with particular ratios of C—N and C—C connected maleimide-derived monomer units which are dependent on the particular initiator used to synthesize the maleimide polymer. The alkali metal polymaleimide salts, which have chelating and anti-scaling properties, are useful as chelating agents and detergent builders and as such are suitable biodegradable replacements for synthetic polymers and sodium polyaspartate.
(2) Description of Related Art
Polycarboxylate compounds, in particular, polyacrylates and its copolymers, are commonly used as dispersants and anti-scalants in water-treatment processes and in detergents (Freeman et al., ACS Symposium Series 626, American Chemical Society, Washington, D.C. pp. 118-136 (1996)). Polyacrylates and its copolymers are primarily used in low-phosphate or phosphate-free detergents to minimize eutrophication of lakes and rivers, which is caused by introducing high concentrations of phosphorus compounds into the water stream.
Annually, several hundred million pounds of synthetic polymers are consumed for their high performance as chelating agents and detergent builders. After use, these synthetic polymers are released into the environmental waters. As a consequence, there is a need to evaluate the effect these polymers have on the environment. The need is particularly acute for those synthetic polymers which are not completely decomposed by natural processes (Freeman et al., ACS Symposium Series 626, American Chemical Society, Washington, D.C. pp. 118-136 (1996)). Because of the low biodegradability of many of these synthetic polymers, much effort has been devoted towards finding biodegradable polymers which can be used to replace the synthetic polymers. One such biodegradable polymer is a derivative of aspartic acid polymers.
The biodegradability, excellent chelating ability, and anti-scaling properties of sodium polyaspartate (SPA) make SPA a suitable replacement for polyacrylic acid. SPA is commonly prepared by the hydrolysis of polysuccinate with sodium hydroxide solution and is a mixture of two isomers, alpha and beta subunits, as shown in FIG. 1. The structure of SPA and the ratio of the two isomers has been determined by 1H NMR spectroscopy (Matsubara et al., Macromol. 30: 2305-2312 (1997); Wolk et al., Macromol. 27: 7613-7620 (1994)).
At present, the monomers which are used to synthesize polysuccinate are aspartic acid, maleic acid, fumaric acid, maleamic acid, or ammonium salt of maleic acid (U.S. Pat. No. 5,393,868 to Freeman et al.; Mosig et al., Ind. Eng. Chem. Res. 36: 2163-2170 (1997); U.S. Pat. No. 5,981,691 to Sikes). However, the process for synthesizing polysuccinate from these monomers is complex and uneconomical because of the high temperatures and the long process times involved. The only attempt to use maleimide as the monomer is disclosed in Japanese Patent No. 65-009394B, which discloses a base-catalyzed process using the maleimide monomer in the presence of a vinyl polymerization inhibitor. However, the process has not been used for the preparation of polysuccinate on a commercial scale because of the complexity of the process and its high cost.
Therefore, a need remains for a simple and low cost method for producing biodegradable polymers which can replace the synthetic polymers currently being used as chelators and detergent builders.